correct syntax for environment paths

Hi,

I've only just started teaching myself how to write cBots - I'm trying out the sample code from 

https://ctrader.com/forum/calgo-reference-samples/54

I'm getting the error "The name desktopFolder does not exist in the current context"

Here is my code:

using System;
using System.Linq;
using cAlgo.API;
using cAlgo.API.Indicators;
using cAlgo.API.Internals;
using cAlgo.Indicators;
using System.IO;

namespace cAlgo.Robots
{
    [Robot(TimeZone = TimeZones.UTC, AccessRights = AccessRights.FileSystem)]
    public class recordData : Robot
    {
        StreamWriter _fileWriter;

        [Parameter(DefaultValue = 0.0)]
        public double Parameter { get; set; }

        protected override void OnStart()
        {
            // Put your initialization logic here
            Environment.GetFolderPath(Environment.SpecialFolder.DesktopDirectory);
            var filePath = Path.Combine(desktopFolder, "cTraderOutput.txt");

            //create file
            _fileWriter = File.AppendText(filePath);

            //file will be saved on each change
            _fileWriter.AutoFlush = true;

        }

        protected override void OnTick()
        {
            // this happens every time a tick happens
            fileWriter.WriteLine("A tick has happened at: " + Server.Time);
            Print("written to file.");
        }

        protected override void OnBar()
        {
            //this happens every time a bar closes
        }

        protected override void OnStop()
        {
            // Put your deinitialization logic here
        }
    }
}

I'm using cTrader 4.1 on windows 10.  I'm guessing it's the environment filepath var that's the problem, I've rummaged around to the best of my ability but I can't find out what I should put in place of "desktopFolder" - please can anyone point me in the right direction as to what the correct syntax should be?

Many thanks in advance

John